1. Field of the Invention
This invention relates to a method of improving process control in the manufacture of acetic acid, and a method of manufacturing acetic acid utilizing improved process control.
2. The Related Art
Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as disclosed in U.S. Pat. No. 3,769,329. This patent discloses the use of a rhodium based carbonylation catalyst, either dissolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. However, it is understood that various catalyst systems, particularly those incorporating Group VIII metals, may be used for the production of acetic acid through the carbonylation of methanol. Generally, the carbonylation reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled. U.S. Pat. No. 3,769,329 discloses that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate, and water concentrations between about 14-15 wt. % are typically used. This is the so-called xe2x80x9chigh waterxe2x80x9d carbonylation process.
An alternative to the xe2x80x9chigh waterxe2x80x9d carbonylation process is the xe2x80x9clow waterxe2x80x9d carbonylation process, as described in U.S. Pat. Nos. 5,001,259, 5,026,908, and 5,144,068. Water concentrations below 14 wt. % and even below 10 wt. % can be used in the xe2x80x9clow waterxe2x80x9d carbonylation process. Employing a low water concentration simplifies downstream processing of the desired carboxylic acid to its glacial form.
U.S. Pat. No. 5,144,068 discloses that, at low water concentrations, there is a synergistic effect between the methyl acetate concentration in the carbonylation reactor and the concentration of iodide salt used to stabilize the rhodium catalyst. It also teaches that an unexpected advantage of operating the reactor at high methyl acetate is a reduction in the formation of undesirable reaction products. In particular, propionic acid is reduced by an order of magnitude. Carbon dioxide and hydrogen, which are formed by the water gas shift reaction, are also reduced.
Various means have been proposed for controlling the processes for the production of acetic acid. For example, U.S. Pat. No. 5,474,774 discloses a system for controlling the liquid levels in a reactor-flasher combination used for the carbonylation of methanol to acetic acid. Liquid level control is achieved by proportional controllers or other controllers, which proportionally change the liquid flow rates from the respective reactor and flasher. An adjustment can be made to the level controllers to change the liquid flow rates by a function generator, which adjusts the flow rate, according to an empirically derived function, which correlates changes in methanol feed rate to liquid flow rates from the reactor and flasher.
European Patent Application EP 1 002 785 A1 describes a process for the production of acetic acid in which the methyl acetate concentration in the liquid reaction composition is maintained at a pre-determined value by monitoring the ratio of methanol and/or reactive derivatives thereof to carbon monoxide being converted to acetic acid and adjusting the methanol, and/or reactive derivatives thereof, feed rate in response.
U.S. Pat. No. 6,103,934 discloses an acetic acid production process with a control process which measures various reactor component concentrations, specifically the active catalyst species, methyl iodide, water, and methyl acetate with an infrared analyzer, and adjusting in response thereto, the concentrations of the catalyst species, methyl iodide, and water to control the acetic acid reaction.
U.S. Pat. No. 6,255,527 B1 discloses an acetic acid production system with a method for controlling the carbon monoxide flow to a reactor by measuring carbon monoxide flowing through a control valve; performing a background calculation to arrive at a time-averaged carbon monoxide flow rate; determining a maximum carbon dioxide flow rate; and controlling the carbon monoxide flow rate so that it does not exceed the calculated maximum flow rate.
All patents and publications referred to herein are hereby incorporated by reference in their entireties.
The present invention relates to a process for monitoring and controlling reactor conditions during the production of acetic acid by the catalyzed carbonylation of methanol. The process of the present invention comprises measuring the density of the heavy phase of the light ends distillation column in the purification system of the carbonylation process. The density measurement is used to adjust the feed of methanol and/or to regulate the temperature in the reaction zone to optimize reactor conditions. The density measurement may also be used to adjust other parameters in the reactor system. The invention is also directed to the system for manufacturing acetic acid based on the process control procedure described.
Monitoring the heavy phase density in accordance with the present invention may be performed near in time to removal of a sample or alternatively, the monitoring may be conducted online. Online monitoring refers to the analysis of heavy phase in real time or substantially real time either by direct insertion of a densitometer probe into the heavy phase process vessel or by rapidly circulating heavy phase process solution through a densitometer and subsequently returning this solution to the process. Off-line measurement refers to the irreversible removal of a heavy phase sample from the process and subsequent analysis being performed on laboratory instrumentation. Further, adjustment of component concentrations and reaction parameters as required should occur substantially immediately following characterization of the sample. This adjustment may be performed automatically in response to the heavy phase density measurement. Finally, it is preferred that the sampling be performed often to minimize undesirable drift from optimum reaction efficiency.
These and other objects and advantages of the present invention shall become more apparent from the accompanying drawings and description thereof.